By the year 1991, the particulate emission standards set by the Environmental Protection Agency (EPA) will require all urban buses to emit less than 0.1 gm/hp-hr of particulate matter. The same standard will apply to heavy duty trucks in 1994. Particulates are defined by the EPA as any matter in the exhaust of an internal combustion engine, other than condensed water, which is capable of being collected by a standard filter after dilution with ambient air at a temperature of 125 degrees Fahrenheit. Included in this definition are agglomerated carbon particles, absorbed hydrocarbons, including known carcinogens, and sulfates.
These particulates are very small in size, with a mass median diameter in the range of 0.1-1.0 micrometers, and are extremely light weight. Particulate filter traps have been developed which are effective to remove a sufficient quantity of the particulates from the exhaust gas of a typical diesel engine for a truck or bus to bring the exhaust emissions into compliance with the EPA regulations. During normal operations of a typical vehicle engine, approximately 20 cubic feet of particulate matter must be trapped per 100,000 miles of vehicle operation. Obviously this particulate matter cannot be stored within the vehicle. Therefore successful long term operation of a particulate trap-based exhaust aftertreatment system (EAS) requires some method for removal of the trapped particulates. One method which has proven to be successful has been to provide a particulate trap for trapping particulate matter, and periodically regenerating the trap to burn off the trapped particles. See for example Mogaka et.al., "Performance and Regeneration Characteristics of a Cellular Ceramic Diesel Particulate Trap," SAE Paper No. 82 0272, published Feb. 22-26, 1982. The regeneration process is typically initiated by a control system and is carried out by the delivery of heat to the inlet of the particulate trap at a temperature in excess of 1200 degrees Fahrenheit. The process results in oxidation of the filtered carbonaceous particulates in a manner that restores the trap's "clean" flow restriction but unavoidably produces temperature gradients and resultant thermal stresses in the particulate trap. The magnitude of these stresses must be controlled to a level that will not result in fatigue failure of the filter within its designed operating life. Due to the complexity and cost of this system, an alternative to particulate traps is needed to achieve up to a 40% reduction in
One such alternative is the catalytic reactor or converter. Noxious elements in engine exhaust emissions may also be at least partially removed by passing the exhaust through a thermal catalytic converter. These converters typically contain a ceramic or metal catalyst support with a precious metal catalyst which will allow chemical oxidation reactions to occur and convert the exhaust gases to a more innocuous form whose presence in the atmosphere is less objectionable.
Catalytic converters are now standard equipment on gasoline powered automobiles, and their practicality for gasoline engines is well demonstrated. Catalytic converters for diesel engines pose different problems which have not yet been solved. Diesel exhaust is cooler than the exhaust from a gasoline engine, especially when the diesel engine is idling or running at low power output. Sometimes the diesel exhaust is so cool that a catalytic converter cannot ignite and burn the easily-combustible carbon monoxide and hydrocarbons in the exhaust. Even when the diesel engine is running at high power output, the exhaust is seldom hot enough to burn the carbonaceous particulates therein. The particulates would pass through the converter and add to the suspended solids in the atmosphere. Therefore, carbonaceous particulates must be controlled within the engine through fuel injection, air-handling, and combustion chamber improvements.
Presently, systems for purifying exhaust gas emanating from an engine include a housing having a chamber filled with catalytic material. The exhaust gas passes through perforated walls or screens into the filled chamber and is discharged therefrom into an exhaust pipe in a chemically modified and more acceptable form. For spark-ignition engines, emphasis has been directed to primarily reducing the oxides of nitrogen in the exhaust gases, while also diminishing the amounts of carbon monoxide and hydrocarbons. Unfortunately, during operation of the engine the amount of nitric oxide in the exhaust gases as well as other constituents vary with the load and other operating parameters of the engine. Also, the overall effectiveness of the catalytic converter varies with temperature changes of the catalytic material. To solve these problems, complex systems have been developed to controllably modify the purification of the exhaust as a function of the temperature of the catalytic material, the engine speed or the load by utilizing dampers, by-pass valves and the like. These complex systems are not only expensive, but the control valves must operate in the very hostile environment of the hot exhaust gas.
One attempt or solution to the aforementioned problem is to utilize different catalyst beds in series as is disclosed in U.S. Pat. No. 3,544,264 issued to Hardison. While the initial catalyst bed may be adjacent the engine exhaust manifold so that it can operate at a relatively high temperature, the next catalyst bed may be located a greater distance from the exhaust manifold where it can operate at a lower temperature. Additional clean air may also be supplied to the beds to promote the reaction. However, there is no consideration for varying engine loads nor is there any provision for shifting the exhaust flow. Consequently the exhaust gas must continuously flow through both converters.
In the U.S. Pat. No. 4,196,170 issued to Cemenska, a multi-stage catalytic converter is disclosed which includes a pressure responsive flow control valve which restricts the flow of exhaust gas through a first catalyst bed in response to a particular operating condition and which allows exhaust gas to flow through both the first catalyst bed and a second catalyst bed in response to a change in the operating condition of the engine. Further, pressurized ammonia is injected into the exhaust flow, so as to reduce nitric oxides in the gas. Preferably, the first catalyst bed is of a material which is effective at low temperatures with the second being of a material which is most effective at a higher temperature, and a third catalyst bed being of a material which is most effective at yet a higher temperature. However, with the arrangement disclosed by Cemenska, the first catalyst bed is continuously subjected to exhaust gas flow even at relatively high temperatures where it is essentially ineffective. By doing so, there is an inevitable likelihood that the first and even second catalyst beds may prematurely burn out because they are unnecessarily exposed to such extreme temperatures.
In an effort to maximize the efficiency of catalytic converters it has been proposed to provide two or more catalytic converters in parallel as is disclosed in U.S. Pat. No. 4,597,262 issued to Retallick and U.S. Pat. No. 4,625,511 issued to Scheitlin et.al. In the former, two catalytic converters of substantially the same construction are disposed in parallel and, during normal operation, exhaust gas is passed through both converters simultaneously. Once a predetermined condition is sensed in either of the converters, fuel is dispersed into the exhaust stream to that converter for regeneration purposes. In the latter, a housing structure is set forth for positioning catalytic converters of substantially equivalent filter efficiencies in series or in parallel wherein the converters are accommodated in a compact housing. However, with these structures, exhaust gas is continuously passed through both converters simultaneously in order to reduce the amount of particulate matter which is expelled into the atmosphere. Neither of the above mentioned disclosures provide for control of exhaust flow between two converters with variations in engine load and more importantly, variations in exhaust gas temperature. The higher the exhaust gas temperature, generally associated with high engine loads, the greater the likelihood of sulfate formations. With the above mentioned exhaust gas treatment systems, there is no distinction made between exhaust gas at low engine loads and that at high engine loads, consequently, the efficiency of such systems can not be maximized over a range of engine loads.